Do you know the five conditions required to grow mold? They are: mold spores, air, food, temperature range between 40F 120F, and relative humidity of more than 80% at the surface. Unfortunately, most of these items are either required for human life and comfort, or impossible to avoid.

Take mold spores, for example. They’re everywhere. Even if you were to kill or remove all the mold spores in a house with disinfectants, air filters and UV lights, every time someone opened a door or window, more mold spores would enter the house again.

Then there’s air. Mold can’t live in a vacuum. Of course, neither can people, so air removal can’t be used to control mold.

Almost anything organic will act as a food source for mold. Drywall, dirt, human food, etc. are all potential foods for mold. If you press your fingertip against a windowpane, there is sufficient oil in your fingerprint to provide food for mold growth. People and mold are both simply part of the organic environment, and we both exist within the same temperature range in that environment.

Therefore, the only reasonable thing that we can do to prevent mold growth is to control relative humidity. Fortunately, as skilled HVAC service technicians, we can do that.

In order to keep the air at all surfaces below 80% relative humidity (RH), we should keep the air in our customers’ homes between 40% and 60% RH. Fortunately, as skilled HVAC contractors, we can do just that.

What to Watch For
There are many conditions that increase humidity in a home. Here are some to watch out for:

Air conditioning duct leakage and boot leakage. If a home’s ducts are in the attic or crawlspace and they leak air in or out (which is likely), air will leak into or out of the house in order to compensate. For instance, if a supply duct system leaks 100 cfm of air into the attic, that acts like an exhaust fan. Any time the home’s heating and cooling system runs, air is being removed from the house, pulling the house into a vacuum. Air will then leak back into the house through any crack or opening it can find. Such openings include fireplace flues, cracks around doors and windows, as well as openings around pipes and wires leading into the house from outside.

If there’s an air leak in the return duct system, air from unconditioned spaces is brought into the house. This is especially true with a dirty or restrictive air filter, which restricts normal return airflow and increases the amount of air flowing through the duct leak. With a return duct leaking air into the duct system, the house is pressurized and leaks air back out of the house through any crack or opening it can find.

Leaking ducts waste the capacity of your air conditioning systems. They not only increase your customers’ utility bills, but can reduce the effective cooling and dehumidifying capacity of your system.

Other mechanically derived or blower driven air infiltration. These include bathroom exhaust fans, kitchen stove hoods, clothes dryer exhausts and window fans. • No return air path. A special case of duct leakage is found in rooms with no return air path. “Florida rooms” or sun porches are common examples. These rooms are commonly separated from the rest of the house by a sliding glass door, and one or two supply ducts are grilles are installed to condition the room.

What is forgotten is that the air has no way to get back to the return grille. The supply air pressurizes the room. Some of this air leaks back into the house through the cracks around the sliding glass door, the rest leaks outside through the cracks around the windows and doors to the outside walls. Because air is leaking to the outside, the main house develops a negative pressure.

Note that if a house has a single, central return, then closing interior doors while the air conditioning is running is almost like having a room with no return air path.

ADVICE FOR HOMEOWNERS Here are some tips you can give to your customers to help them keep relative humidity levels inside their homes reasonable:
• Don’t operate the air conditioning with any windows open. This includes opening bathroom windows when taking a shower or to reduce odors. Yes, this brings in fresh air, but humidity in the air will condense onto the registers that are nearest to the open window. If you must open a window because the bathroom doesn’t have an exhaust fan, close it as soon as visible moisture on the walls and mirrors is gone.
• Don’t run the indoor blower on continuous operation. If you do, any water left on the coil or in the drain pan will re-evaporate when the compressor shuts off. This can raise the relative humidity in the house by 15%.
• Don’t set the thermostat too low. If you have the temperature in the house too low, you increase the risk of condensation forming on some surface in the house. This could even occur on a hidden surface, such as inside the walls.
• When operating exhaust fans, remember that while you’re exhausting air and moisture to the outside, you’re also bringing air and moisture back in from the outside. Whatever you blow outside will leak back in somewhere else. Don’t run your exhaust fans any longer than necessary. This includes bathroom exhaust fans, kitchen exhaust fans, range fans, and clothes dryers. If the air outside has more moisture in it than the air you’re exhausting to the outside, you’re doing your house more harm than good.
• Make sure exhaust fans terminate outside. I’ve often seen dryer ducts that terminate in the crawlspaces and attics. Many bathroom exhaust fans blow into attics. This can cause tremendous moisture problems in attics and crawlspaces. Your air conditioner cannot handle this kind of problem.
• Don’t pile ground too high against house. Water can wick into slab edge, foundation, and crawlspace. Waterproof the edges of the slab or the below-grade foundation walls. Put moisture barriers on top of all foundation piers. Don’t let wood contact ground or bare concrete.
• Consider installing a dehumidifier in problem areas of your house, basement or crawlspace. Keep in mind that there are whole-house dehumidifiers that can be connected to your air conditioning and heating duct system that will dehumidify your house extremely effectively, thus improving both your air conditioner’s performance and your comfort.

• Home infiltration from natural building leakage. This includes crack leakage around doors and windows as well as electrical outlets and other building envelope cracks. Plumbing pipes coming up through the floor from a crawlspace can also leak air into the house. This infiltration is mostly winddriven. Leaving windows open during cool mornings in the summer has the same effect. Even though the outside air temperature is low, the moisture levels in the outside air can be very high.

If the house has a damp crawlspace or basement, water vapor can penetrate through wood floors into the house. Water vapor flows from high concentrations to low concentrations, just like heat. This is in addition to air infiltration already described above.

Improperly sized and improperly installed air-conditioning systems. Significantly oversized air conditioners and heat pumps do not control humidity as well as properly sized units. This is because the indoor coils do not get really cold until after the cooling unit has been running for as long as 15 minutes or more. Oversized units tend to short-cycle: they cool the air quickly, and then shut off.

What You Can Do
There are many steps you can advise your customers to take to keep RH levels inside their homes reasonable (see the sidebar, “Advice for Homeowners”). And there are a number of things we can – and should – do to ensure that our systems aren’t part of the problem.

First, when installing central air conditioning in a house, attention must be paid to several factors. The first factor is equipment size. It is common practice in most parts of this country to install one ton of air conditioning for every 500 sq.ft. of floor space. This rule of thumb should be discarded.

It has long been known that oversized equipment does not dehumidify as well as properly sized equipment. This is because it takes as long as 15 minutes for the indoor coil to get really cold. In order for moisture to be removed from the air, the air must be cooled below its dew point. The dew point is where the RH is at 100%. The colder the coil, the more moisture that is removed. Always do a proper load calculation.

The second factor is system airflow. In humid climates, the airflow should be approximately 350 to 400 cfm/ ton. If the airflow is too high, the coil does not get cold enough to remove enough moisture. If the airflow is too low, your system loses capacity and efficiency, increasing utility bills and reducing comfort. Always measure your systems’ airflow.

The third factor is sensible heat ratio. The sensible heat ratio is the ratio of heat that is removed as temperature that can be measured by a thermometer compared to the total heat removed. Total heat removed also includes the Btu removal required to dehumidify the air. The lower the equipment sensible heat ratio, the better the dehumidifying capacity of the cooling equipment. The load calculation will tell you what the sensible heat ratio of the house is. Your equipment’s sensible heat ratio should be equal to or below that of the house.

The fourth factor is duct leakage. If the ducts are not in the conditioned space, any air that leaks out of them or into them affects the pressurization of the home. This can increase infiltration of moisture, dirt and mold spores into the living areas. Sealing any ducts that run through unconditioned spaces will reduce this infiltration and improve air quality.

Seal around all openings at grilles and registers. This helps keep the humid outside air from condensing on the surfaces of the grilles and around the grilles.

Discharge air conditioning condensate outside. Do not allow condensate to drain into a crawlspace.

The bottom line for our industry is that we can help homeowners control their homes’ humidity levels, thus enhancing their comfort and preventing mold growth. Is there any reason we shouldn’t be doing this on every job?